Like a lot of folks, I have a bunch of gadgets that I just don't really use that much. Right now, I have two kitchen drawers filled with stuff. I used to have 3 - 4, but I pitched a lot of stuff when I moved to MI.
Right now, I'd have to say my most useful gadgets are:
Wire whisk
Flat whisk
Probe thermometers
silicone spatula (used for eggs and omelettes)
The small food processor attachment for my immersion blender (I've used that attachment more the immersion blender itself)
My KA mixer, which, unfortunately, isn't working right now. Due to other problems, I haven't had the time to see where the loose wire is.
I also have a couple China Caps, one of which is commercial-strength. I use it like Lugaru mentioned, to fish out solid materials from liquid (usually bones from broth), then, with the hook and handle of the China cap, mount it on a stock pot and pour the broth through to remove the last of the solid materials.